A semiconductor layer transfer method called SMART-CUT® is known to those skilled in the art. Details of this method may be found in many published documents, for example, on pages 50 and 51 of: “Silicon on Insulator Technology: Material to VLSI, Second Edition,” by Jean-Pierre Colinge, published by “Kluwer Academic Publishers”. A second step of bonding the host wafer (the recipient of the transfer layer) typically takes place on the surface of the donor wafer and includes using a bonding layer made of dielectric material such as SiO2. In this manner a semiconductor-on-insulator structure may be formed, such as an SOI structure (in the case where the transferred layer is made of silicon), SiGeOI (when the transferred layer is made of germanium silicon), sSOI (when the transferred layer is made of strained silicon), or GeOI (when the transferred layer is made of germanium).
During the detaching step, thermal energy typically is at least partially utilized. In this case, the thermal budget (the combination of temperature and duration of the heat treatment) must be considered to determine the moment at which the transfer layer will be detached. It has been observed that after detachment of the transfer layer, the latter may have quite a rough surface, as well as a lower quality crystalline surface structure, resulting from the previous implantation and detachment steps.
FIG. 1 illustrates a semiconductor-on-insulator structure 30 that includes a host wafer 20, an electrically isolating layer 5, and a transferred layer 1. The semiconductor part, which is the transferred layer 1, has a reduced crystalline quality surface. In particular, the transferred layer 1 includes a defective zone 1A that has crystalline defects and a detrimental surface roughness. The defective zone 1A typically has a thickness of around 150 nm, and the implantation step may have caused a reduction in the crystalline quality in the transfer layer 1. It is thus necessary to treat the transfer layer 1 to remove the defective zone 1A, and thus to reclaim at least part of the sound zone 1B of the transfer layer 1. Typically, the defective zone 1A is oxidized and then subsequently removed by chemically etching using hydrofluoric acid HF (a treatment called sacrificial oxidation). A finishing step is then used, such as mechanically polishing or chemically-mechanically polishing. Such a treatment step for the transferred layer 1 is costly and complex.
The defective zone has to be completely removed after detachment, in particular due to the defects present within the transfer layer. It is thus usual to transfer a layer of greater thickness in order to completely eliminate these defects during finishing operations performed after detachment. For instance, the formation of a structure comprising a 500 angstroms thick taken-off layer necessitates the transfer of 2000 to 2500 angstroms and the removal of 1500 to 2000 angstroms, for instance by polishing, selective etching or sacrificial oxidation.
The treatment of the transfer layer 1 is therefore classically performed to remove the defective zone 1A, and thus recuperate at least part of the sound zone 1B of the layer taken 1. Typically, oxidation of the defective zone 1A is first conducted, then this is subsequently removed by means of chemical etching using hydrofluoric acid HF (thus creating a treatment called sacrificial oxidation) then finishing, for example by mechanical polishing or chemical-mechanical polishing. Such a treatment step of the transfer layer 1 is consequently costly and complex from an economic point of view.
Furthermore, the use of such treatments during finishing operations requires the systematic removal of the negative of the donor wafer to obtain access to the surface of the transferred layer 1. The wafers must therefore be removed from the furnace (in which heat treatment was conducted). This results in a loss of time, extra wafer handling and the need to use suitable equipment.
US patent application 2004157409 describes a method that attempts to overcome these problems, by including a stop layer between the future defective zone 1A and the underlying future sound zone 11B. In this example, the transfer layer is made of SiGe and the stop layer is made of Si. This publication also teaches to improve the finishing operations by using selective double etching (of the defective zone 1A and the stop layer), to substantially reduce the roughness as measured from their maximum values (peaks and valleys) and depending on their quadratic values (in RMS Angstroms) on the surface of the SiGe sound zone 1B.
However, selective etching is imprecise, and thus a roughness remains at the tip or edge of the first chemical etching step which is at the interface between the defective zone 1A and the stop layer. Chemical etching therefore unequally treats the surface of the stop layer. In addition, since the stop layer is generally fairly thin, the first selective etching process can pass through it and attack the underlying sound zone 1B. It has also been proposed to polish before selectively etching, in order to eliminate some of these potential problems. However, the combination of polishing and selective etching further adds to the complexity and cost of the operation, which could make the overall operation unprofitable.
In U.S. Pat. No. 6,953,736, the transferred layer 1 is composed of a sound zone 1B made of strained Si and a defective zone 1A of SiGe. The defective zone is selectively removed from the sound zone 1B. A similar problem exists here analogous to that discussed above, wherein the surface of sound zone 1B is unequally etched because the tip of the etching portion reached it.
It is further mentioned that it has also been proposed to perform the implanting with several atomic species (typically by performing an implantation of helium and an implantation of hydrogen). This type of implantation is hereafter designated by the term of co-implantation. Performing a co-implantation indeed allows for a total dose of co-implanted species to be used lower than when a single species is implanted. The total dose in co-implantation thus typically represents ⅓ of the dose in single species implantation. It results from this decrease in total implanted dose a decrease of the defective zone thickness which in particular allows to decrease or to simplify the finishing treatments performed after detachment.
In addition, U.S. patent application Ser. No. 11/181,405 filed Jul. 13, 2005 discloses, with relation to co-implantation within a Si layer, the co-implantation parameters that help minimizing roughness lead to the formation of certain defects (such as blisters at the bonding interface, or crystalline defects within the thickness of the taken-off layer), and reciprocally the co-implantation parameters that help minimizing said defects lead to increased roughness.
In other words, a compromise has to be made between roughness and formation of these defects, and the recourse to co-implantation thus does not allow as such to satisfactorily solve the above mentioned problems relating to the presence of defects and a superficial roughness. The present invention now provides a commercially acceptable compromise for these variables.